Tetrahydropyridine

Tetrahydropyridines (or piperideines) are heterocycles with the formula C5H9N. Three isomers exist, which differ by the location of the double bond. None of the parent species occur widely, so they are mainly of theoretical interest. Although the parent tetrahydropyridines are rare, many substituted tetrahydropyridines are known.

Preparation and occurrence
2,3,4,5-Tetrahydropyridine, a colorless liquid, is commercially available. It is an imine.

Illustrating another isomer of tetrahydropyridine, 6-acetyl-2,3,4,5-tetrahydropyridine occurs naturally.

Partial reduction of pyridinium salts gives N-alkyltetrahydropyridines. Treatment of N-methylpyridinium with borohydride reagents gives 1-methyl-1,2,3,6-tetrahydropyridine. A modified Ireland-Claisen rearrangement leads to tetrahydropyridines via a silyl ketene acetal intermediate. Ring-closing olefin metathesis has also been used to establish the tetrahydropyridine ring system.